23 Days, 23 Years
by AgentMaryMargaretSkitz
Summary: (Hybrid Child Universe) The Doctor and Clara lose their son while visiting a planet. Now trapped in another world, Leopold Fitz Oswald-Smith is alone, and tries to find his way through life as Leo Fitz. The story of how he grew up while his parents searched for him.
**So when the Oscars were going on and people were going "Oh DiCaprio's gonna get it this year, yes!", I was shaking my head and thinking it wasn't going to happen. I was so sure of this that I posted on Tumblr that if he won, I would write a fanfic on the request of the first person who sent one into my ask box. As you all know, Leo won, and the request was a Hybrid Leo Fitz fanfic. I was happy about that and got an idea, scrapped that idea, and thought of a new one. I only meant to write a little short one-shot.**

 **Turns out it grew a mind of it's own. It's something that I had thought of in the early days of this theory regarding how Leo might have grown up. It's quite a bit AU near the end, but it's actually a bit of a nicer turn of events for Jemma and her return in my opinion.**

 **So this is the result. And I'm very proud even though I cried several times writing it.**

* * *

Day 1/Age: 5

They'd never been to this planet before, which was exciting. The sky was dark, but not enough that they needed a torch or anything. There was more gravity too, according to his father. Leo noticed it when he jumped up and down.

"It's been uninhabited for a long time," his dad was telling Mum as they walked up a hill. "But there are ruins here of an ancient civilization."

"What happened to the people who lived in them?" Mum asked. "It's safe for Leo, right?"

"There's no danger. The ruins are centuries old, Clara. We'll all be fine."

Leo stopped listening to them around that point. He bounced a rubber ball on the ground, dropping his hand a little lower to catch it when it came back up. When he threw it down again, it hit a stone and ricocheted down the hill. Leo watched as the ball bounced down the steady decline into the valley below.

"My ball!" he cried out, jumping from the path and running down the hill toward it.

"Leo, careful!" His mother called out. "You don't know what's down there!"

He ignored her and hurried toward his ball. Mum had a tendency to worry a lot more nowadays. But Leo was sure that he'd be fine.

He smiled as he bent down to pick up the ball. As he did so, the surface below him trembled. His hand snatched up the ball up quickly when the tremor hit him. Before he could run, the ground below his feet seemed to disappear as a hole opened beneath his feet.

Leo heard his parents scream his name before he fell through.

* * *

"No!" the Doctor shouted as his son disappeared into the hole that had appeared in the ground. "No!"

He scrambled down the hill toward it as it started to close. By the time he had dropped to his knees beside it, the hole had closed. His hands sifted through the sand, searching for something that had caused it. Nothing was to be found other than sand.

"Doctor…" Clara was now beside him. "Leo…"

He pulled out the sonic and started to scan the ground.

"Doctor, what happened?"

The reading came through.

"Doctor, where is Leo?" Clara demanded, sounding on the verge of tears. "Where is our son?"

He stood up, looking at the spot numbly. "It seems to have taken him somewhere."

"Where though?"

"I don't know," he mumbled. His son was gone. "I don't know. Clara, our son…."

"No, don't even think that!" She shook her head. "He can't be. He's only five, he's too young. Leo's got to be out there! He can't be…gone. He has to have gone somewhere."

The Doctor pulled her close into a hug, hiding his face from her.

He'd never felt failure like this before.

* * *

Age: 5

Leo landed on something hard in the midst of a strange dark grey substance. It started to recede behind him, and he turned to see a large stone of the same color rising up and forming before his eyes. Climbing to his feet, he noticed that he was in a glass box in a dimly lit room. He ran around the stone, trying to figure out what had happened. His parents were nowhere to be found.

"Mum?" he whimpered slightly, moving to the glass and peering through it. "Dad?"

No one else seemed to be in the dimly lit room other than himself.

The stone! It must have been a portal or a transport of some sort.

Leo moved up to the stone. "Bring me back!"

The stone did nothing.

"Bring me back!" he repeated, trying to shake it.

Still nothing. Leo was beginning to feel scared and upset now.

"Bring me back!" he yelled, pounding on it with his fists. "Bring me back!"

Nothing was happening. Leo's hands started to hurt after a while from hitting the stone. He slumped down against it and started to cry.

He didn't know how long he cried for. A few days ago, he'd misplaced the watch he'd gotten for his fifth birthday, the one with the monkeys on the wristband. Leo didn't wear it a whole lot, but it was still cool. At least he still had the sonic screwdriver his dad had made with him.

The sonic!

Leo dug in his pocket for his sonic screwdriver. It was nowhere as large as the one his father carried around and didn't do nearly as much, but he still loved it anyways. Standing up and moved toward the door of the box, Leo raised the sonic and thought 'open'.

The door opened outward with a small click. Leo put the sonic back in his pocket and hopped out onto the floor of the room. He shut the door behind him once he was out and walked to the door at the end of the room. There was no handle, so he withdrew the sonic again and repeated the process he had completed with the door. It took longer for this one, but he managed to get out.

Leo repeated the process with every door he came across until he was outside. The layout of everything reminded him of the UNIT headquarters he'd been in a few times before, but he didn't spot the UNIT logo anywhere. That meant if he was caught, they probably wouldn't know that he was half alien and would try to experiment on him. Aunt Nelly had told him that other organizations did that one time when he went for a check-up. With that in his mind, he wanted to run.

He heard voices coming his way. Leo ran across the pavement until he found a car. Using his sonic, he opened the trunk and climbed inside. Once he was in, he reached up and pulled the back down, plunging his world into darkness.

Leo curled up tightly in the dark of the trunk. The ball that he had been playing with was pressing up against his leg. He was starting to miss his mum and dad a lot.

" _What if they don't find you?"_ a mean voice said in the back of his head.

"They will," he mumbled out loud in reassurance to himself, yawning a little and closing his eyes.

* * *

He woke up when his head hit the side of the trunk. Leo tried to sit upright, but only succeeded in hitting his head again before remembering where he was. He felt himself shifting around a lot, and realized that the car was moving. The owner must have come along and was now driving somewhere.

When the car stopped, Leo used the sonic to open the trunk and jumped down onto asphalt. Rain pelted his face as he took notice of other cars that were stopped around him waiting for the light to change. They looked different than the ones that he had seen in Mum's time. The buildings around him looked different too. He must have arrived in a city, judging by their height.

The rain started to pick up, and Leo raced across the road, taking refuge in an alleyway.

* * *

Day 2:

They'd brought the TARDIS closer now so it sat on the hill above the valley where their son had vanished. The whole night, they had waited to see if Leo would reappear or if anything would happen. Clara couldn't remember falling asleep, but she woke up with a blanket wrapped around her, her head resting on the Doctor's shoulder.

"How long was I out?"

"Eight, nine hours."

She nodded, rubbing at the dried tear tracks on her face. Checking her watch, Clara realized they'd been on the planet for over a day now. Leo had been gone for just that long. The ground where he had fallen through was no different.

"He's not underground, is he?"

"The sonic just registered dirt, no traces of life."

"How about the rest of the planet? Maybe it took him somewhere else on here?"

* * *

Age 6:

The police had finally found out he'd been living on the streets. It was two months before his seventh birthday when it happened. The loaf of bread that he had stolen from the bakery was what did him in. Normally, he could slip in unnoticed and steal something off the racks by the door so he could eat for a few days. But this time, the owner spotted him and grabbed him by the arm. If he'd had his sonic, then he could have used it to get away, but it was back in the abandoned building that he was currently calling home.

When the police arrived, they took him down to the station after he told them he didn't know where his parents were. Once they arrived, one of the officers sat him down in a chair and asked about them. Specifically, who they were.

"Clara Oswald and Doctor John Smith," he told them, using his father's alias like he'd been instructed to in the case he was ever lost.

"Where do you live?"

"London, England."

The officer wrote this down and left him the room, shutting the door behind him. As soon as he was gone, Leo tried to open the door, but it had been locked as well. He ended up sitting on the chair and drummed on the table. He was concerned, as he had learned over the past year that he had landed in Glasgow, and the year was currently 1993. His mother was only a child, but his father might be able to be contacted.

When the officer returned, he didn't have good news. There was no Clara Oswald in Glasgow or London. There was a Doctor John Smith in London, but he looked nothing like Leo's father, or any of his father's previous faces. It was disappointing, as he'd really hoped that it had been his father.

"When was the last time you saw your folks?" the officer asked him.

"Over a year ago."

They had always told him they would find him no matter what happened. They promised that, but they still hadn't come.

A few hours later, he was put in an orphanage. He was able to be entirely clean for the first time in a year, and also got his first hot meal in over a year. But they gave him a new name for the legal papers. He was now Leopold Fitz, his middle name now becoming his last. The Oswald-Smith he had gone under at his old school was stripped away.

* * *

Day 3:

Their search of the planet had turned up no trace of their son. And there was still no change in the ground in the valley or anywhere else on the planet.

"We need to expand our search," the Doctor sighed. "I can put sensors on the planet to see if they detect any activity similar to what happened."

"I don't like leaving, but he could be elsewhere. We have to find him, Doctor."

"We will, Clara. I promise that we will find Leo."

* * *

Age 8:

Today was Lucy's birthday, or at least it would be many years from now. Leo missed her almost as much as he missed his parents. She was one of his best friends, and they'd known each other since they were little. His parents had known her father even longer.

Hopping off the bottom bunk, he bent beside his school bag and pulled out an old piece of homework from school. Using his folder as a surface, Leo turned the paper over and began to write a birthday letter for her. This hadn't been the first time he'd written a letter before to someone who wasn't there. Sometimes he wrote little messages for his parents too, even though they still hadn't come yet. But he hoped they would.

He was so focused on his letter that he didn't even hear his social worker, Chris, enter the room, or even notice the woman with dark hair who was with him, until a few moments later.

"Leo here is an incredibly intelligent child," Chris was saying. "Came in off the streets about two years ago after being on his own for a year. But Leo's a sweet child. Quiet, but sweet."

The woman with Chris smiled at him as both noticed that he was now looking over at them.

"Leo," Chris said warmly. "I'd like you to meet someone."

Leo set down his pencil and the letter and walked over to them. The dark haired woman with Chris smiled more and bent down to face him.

"Hello, Leo," she greeted, holding out her hand. "I'm Jane."

"Hi," he replied quietly, shaking her hand.

"Ms. Evans has expressed interest in having you stay with her," Chris told him.

Jane nodded. "Does that interest you? Do you want to come stay with me?"

Leo shrugged. He didn't like staying in the orphanage. It wasn't terrible and the adults were kinda nice. But the other kids, the ones who didn't have parents, bullied him about waiting for his. They made fun of him for a lot of things, whether it was his two hearts that all the doctors were calling a defect or that he had skipped ahead two levels in school above all the other children his age.

"Okay, I guess," he mumbled as both adults waited for their answer.

Hopefully, his parents could still find him if he went with Jane. She seemed nice enough to stay with for a while. Leo still clung to hope that his mother and father would find him. But after three years, he was starting to wonder if they were still looking for him, or if they would even come back.

* * *

Age: 10

Leo sat on the floor of his room, sniffling. He was still with Jane after two years, and he'd grown to like living with her. But he was still waiting for his real parents to come and get him. Five years had passed since he fallen through the hole and landed in the past, and yet there was still no sign of them. He didn't know why they hadn't come.

Looking up at his bulletin board, he gazed between the various awards that were pinned up at the picture of his family that he had drawn a while ago. It was no work of art, but it was a close enough illustration to remind him of what they looked like, even though he could still remember them. Usually he smiled when he saw it, but today he frowned.

The other kids at his school teased him a lot, just like the kids at the orphanage had. He was mocked by the students in his class for being a 'baby', and the children his age wanted nothing to do with 'Weird Leo'.

"I'm not weird!" he would shout.

"You're a freak!" they said to him. "That's why your parents dumped you."

"I'm not a freak! And they didn't leave me!"

"They left you. It's obvious."

"They said they'd always find me! My mum and dad are coming back!"

"They lied, freak. They don't love you. Your parents are never coming back!"

Leo wiped his hand across his eyes. If his parents did love him, then why hadn't they come? The TARDIS could travel through time, surely Mum and Dad could have found a way back to him. He had tried his best to leave signs, writing his full name on bus benches and anywhere else with graffiti. Sometimes he drew inventions he knew about from the future and wrote the current date he was living in, hoping that maybe someone would find it somewhere and his parents would find out about it.

What if the other kids were right? What if they weren't ever coming back?

Leo stood up and snatched the drawing off his bulletin board. He tore it into pieces and threw the scraps into the wastebasket. Then he found all the messages and little letters he'd written to his parents, to Lucy, even the ones to his aunts and uncles, and ripped them apart.

His parents were never coming for him. He should have realized that a long time ago like the other kids had.

* * *

Day 7:

A week after Leo had vanished, the Doctor and Clara went to the Paternoster Gang. All three of them, even Strax, were astonished at the events that had occurred on the planet. They were Leo's aunts and uncle and all loved the boy dearly, even though Strax needed to stop sending various weapons for Leo's birthday. Vastra, Jenny, and Strax all had special places in their hearts for Leo, and promised to help with the search in any way that they could.

"You will be reunited with your son," Vastra told Clara as she and Jenny entered the TARDIS.

"And the boy's captors shall be mercilessly destroyed!"

"Not if I get to them first," Clara muttered under her breath. Whoever was responsible for her child's disappearance would be facing her wrath.

* * *

Age: 12

Leo ran into the house, slamming the door shut behind him. "I'm home!"

"Leo, we talked about the door!"

He entered the kitchen sheepishly. "Sorry about that, Mum."

His adoptive mother shook her head, although Leo could see that she was smiling. Jane had officially adopted him about a year and a half ago, and he'd started to call her Mum shortly after he'd given up hope of his birth parents returning. They'd probably given up on him first, and he should have realized that sooner. Jane might not be his birth mother, but she had raised him well.

"How was school?" she asked, setting down her pen and looking up at him.

"School was good."

"You're eager to get back to that doohickey you've been working on, aren't you?"

"It's not just a doohickey!" he said excitedly. "It's going to help regulate the heat that goes through the burners on the stove so it finally works on all the burners and not just the one."

"I know it well," she nodded before standing up and holding out an enveloped for him. "And also, this came in the mail today for you."

Leo took the envelope and flipped it over. It was from MIT. Cracking it open, he took out the papers inside, a smile spreading across his face.

* * *

Day 12:

"Where is my son!" the Doctor demanded, producing his sonic from his pocket. "Where is Leopold Oswald-Smith!"

"We do not have the child," the torso and head of the Cyberleader responded. The rest of his limbs had been separated from him, courtesy of Jenny and Clara.

"We found your ship to be located nearby the planet he vanished on," the Doctor shouted, holding the sonic up to the control panel of the ship. "Your ship has a teleportation unit. You used that to take him, didn't you?"

"We do not have the child."

"Did you use the teleportation unit to take him?" Clara yelled angrily.

"The unit," Vastra announced, entering the room with Strax. "Is not operational."

As she said those words, Clara looked over at the Doctor. Her large eyes looked full of despair.

"What are you talking about?"

"The Cybermen's teleportation unit is broken, and it's been that way for a long time," the lizard woman explained. "There is no way they could have taken him. Strax checked the cells too for good measure. No sign of Leopold."

The Doctor sighed in relief that his son was not with the Cybermen, but the feeling lasted a second before he remembered that Leo was still lost.

* * *

Age: 17

"The title's different from the one on the reruns."

Leo was barely paying attention to his best friend. He was staring openmouthed at the television. Jemma had dragged him into watching some revival of a show that she had seen a few times when she was younger. Leo had never heard of it before, but apparently it was quite popular back when it first aired.

What he wasn't expected was to see his father's TARDIS spinning through what he assumed was supposed to be the time vortex. The box was a bit of a more faded shade of blue, but he immediately knew it was the TARDIS. Jemma had told him the show was called Doctor Who, but he had never made the connection. Now, he was finding out there had been a show with his dad all this time, even though it had been over a decade since he had last seen his parents.

"Doctor Who?" he murmured aloud as the title came up.

"Shh," Jemma nudged him. "It's starting."

Leo didn't say a peep for the rest of the episode, instead taking in everything that was playing out in it. He knew all these people were actors, but he had seen a picture of Rose and Jackie and Mickey with a bunch of other people who had once traveled with his father. They were exact twins. It was a bit puzzling to him, but maybe it was only a coincidence.

As he watched the Autons near Rose (and shivered slightly as he recollected an encounter with them when he was four), Leo frowned a bit at the dramatics. But it looked as though Rose was going to die. Maybe things were different in this show. Perhaps Rose never did travel with his dad.

A hand grabbed onto Rose's. When the camera showed the identity of the person, Leo couldn't help but flinch slightly. The actor was a dead ringer for one of his father's younger faces, the one his mother had called Big Ears.

Leo curled his hands into fists as he thought about his parents. Twelve years had gone by since he last saw them. He had spent years believing that they were coming back. When he was still living on his own, he would climb up the roof of the abandoned building he had taken shelter in and held up his sonic to the night sky. He would send off a few pulses into the air, hoping that his parents would lock onto the signal and find him. But those days were long gone, and Leo knew they were never coming back.

As angry and upset as he was with his birth parents, the show that Jemma was watching with him was actually interesting. Maybe he could learn more about his birth father if he watched it. His birth mother too, if she showed up at all.

* * *

Day 15:

They dropped Vastra, Jenny, and Strax back off in Victorian London. Two weeks had passed for the Doctor and Clara, and there was still no sign of their son. Leads were running dry, and their friends were losing hope. Clara had talked with the Doctor about them, and they both agreed that they couldn't keep the Paternoster gang from their own lives and time any longer.

"Good luck," Jenny told them before they left. "I do hope you find him."

"As much as I would have liked to do more, I fear he is lost," Vastra said to Clara in private moments before.

Clara had shaken her head at that. "Nothing is lost forever."

* * *

Age: 20

"You're an alien?"

"Technically, half alien," Leo corrected as Jemma continued to gape at him. "I'm half human on my mother's side. But my dad wasn't human. That's not even the weird part."

"And what's that?"

Leo took a deep breath. Jemma was the second person he'd told this to. The first had been Director Fury when he had first joined SHIELD and been confronted by his medical file. Jane and the doctor in the town they had lived in knew he had a second heart, but the doctor had determined that it wasn't harmful to his health and nothing more was done on that matter. In addition to that, there had been a security tape of his five year old self escaping from the SHIELD facility that he had found himself in. When he saw the footage, Leo closed his eyes and cursed his younger self for not considering that there were cameras around. He could have easily wiped the footage with his sonic.

He explained the whole situation to Fury twice, once when he was first questioned and a second time where Leo was hooked up to a lie detector. The Director had asked if there were any more of people like him, which there weren't. It must have reassured Fury, along with his clean test on the lie detector, because he allowed Leo to join SHIELD. He did warn him that he would be keeping a close eye on him.

Leo hoped he was making the right move telling Jemma. He did feel that his best friend could be trusted though. The two of them had known each other for a long time, and while he knew plenty about her, she knew little about the real truth that surrounded him.

"What else is there, Fitz?" Jemma asked. "Has something happened?"

"No," he shook his head. "Something did happen a long time ago to me though."

Jemma sat back in the chair, waiting for him to continue.

"When I was five, I was on another planet. Something…happened with the ground and I ended up on Earth in the nineties. For years, I thought that I had gone back in time, but then you showed me Doctor Who a few years back."

"You're not the Doctor, are you?"

"No, but he's my father," Leo admitted as Jemma's eyes widened. "But after you showed me that show, I think that I didn't go back in time. I might have landed in a completely different universe than the one I was born in."

"So this is an alternate universe to you?" Jemma murmured, looking up at him. "And Doctor Who isn't a show in the one you're from? It's actually real life?"

Leo felt his shoulders slump. "You don't believe me, do you?"

"No, I do," his best friend protested, standing up. "It's just a lot to take in."

He could see that she was telling the truth. She did believe him.

"You and Director Fury are the only other people who know about this, Simmons. You have to keep it a secret."

"I promise," she nodded. "But I have one question."

"Which is?"

"What about your parents? You said you were five when you came here, you must have been with them at some point before that. Are they not able to travel between the two universes? Is that why you were adopted?"

That was a question he had been asking himself over the past year. "It's been fifteen years since I last saw my parents. They probably gave up any hope of finding me years ago. After all, I did too."

* * *

Day 17:

Leo's room was starting to feel dead. The Doctor had walked past it every day since his son had disappeared. Every day, there was a moment of panic when he saw the sight of the empty bed and unmade sheets before he remembered Leo was missing.

He'd lost children and grandchildren before. When Clara found out she was pregnant, he swore he would never let history repeat itself again. They had both done what they could to keep Leo safe, but something had still happened. And the Doctor had failed to protect one of the people he loved.

As he stared into the empty bedroom, Clara appeared beside him. She didn't say anything.

"I'm sorry," he said aloud. "I should have never taken us to that planet. Losing Leo…it's all my fault."

"This is my fault too," Clara murmured. "You don't have to take all the blame."

They were silent for a while before Clara sighed and shook her head. "I should have done something. He probably thought he was fine, that's why he didn't listen to me. I wish I had held him back or gone after the ball myself. And now I'm scared he's gone forever."

"Clara, we're going to find him," the Doctor promised. "I won't ever stop looking."

"Neither will I. Not until he's back."

* * *

Age: 24

Leo stared at the screens in horror as videos of the alien invasion streamed through SHIELD's communications. Jemma was beside him, her arms hanging limply at her sides. Her eyebrows were knit with concern. People in the room with them were experiencing a variety of reactions. Some were crying, others praying, but most were just watching in stunned silence like him and Jemma.

A cheer went up as Captain America took out some of the invaders.

"Have you ever seen anything like those before?" Jemma whispered beside him.

Leo shook his head. His Earth, the one that he had spent the first five years of his life on, had seen alien attacks before. He had learned about them from his parents. They'd told him about the invasions of the Sycorax, Daleks, Cybermen, and all the others. He even learned about the Year that Never Was from Martha Jones, who had babysat him with her husband a few times. But the things invading New York were a species he had never heard about.

For a fleeting moment, Leo found himself wishing for a blue box to appear, but brushed the thought aside a moment later.

They were never coming back.

* * *

Day 21:

They decided to return to the planet. Maybe they had overlooked something three weeks ago. Perhaps they could find something if they searched again.

"What if he thinks we've given up," Clara asked him that night. "It's been three weeks. That's such a long time for a five year old."

"I don't think so," the Doctor replied, shaking his head. "We've always told him never to give up. And we promised that if anything ever went wrong and we were separated, we would always find him."

* * *

Age: 25

Watching Doctor Who together was now a tradition for him and Jemma. It was interesting to see the stories each week, and how they all were similar to stories that he had heard at bedtime when he was still with his parents. Jemma would ask him questions from time to time about the characters, as well as the Doctor. The previous year, she had asked if River Song was his mother. He'd shaken his head at that and told her that he had never met the woman (although she seemed interesting), and that he'd yet to see his mother. There had been an instance a few years back where there was an actor on the show who looked like his father.

Finally, she appeared, or at least the actress did. But when he first saw her, her character was called Oswin. But after Oswin died in the episode she was introduced, he started to wonder if maybe the story was taking a turn. Everything else on the show matched the bits and pieces he knew about his father, but this was different.

At Christmas, she appeared again, except it was set in Victorian England. This time, she was called Clara. But she died again, her last words being the exact same as Oswin's. Leo realized it was the new mystery of the season.

Spring came. And finally the Doctor met the original Clara Oswald.

"So that's your mum in your universe?" Jemma asked when she saw him smile.

Leo nodded.

* * *

Day 22:

Nightmares troubled Clara even more now. She had been afraid of losing her son before. Now she feared that she and the Doctor would never see their son again, or that something terrible would happen to him.

The Doctor woke her up from her latest nightmare that morning. Apparently she'd been screaming.

"I saw him drowning," she explained. "I could barely see him, but he couldn't swim. And I couldn't save him."

"It was only a bad dream, Clara. It wasn't real."

She nodded slowly, although she felt a strange sense of dread for a few seconds later on in the morning that she couldn't explain.

The Doctor felt it too, but he mentioned nothing of it.

* * *

Age: 26

He was only able to make one mask to deliver one burst of oxygen. His heart sank as he realized this. Sighing, Leo cast a look over at Jemma before staring back down at the mask. They'd done so much together, and now only one of them could make it out.

There was no hesitation when he understood that it would be her.

When he was little (and still with his parents), Leo remembered that bedtime stories were a strong tradition. He heard about so many fantastic tales and adventures, and couldn't wait to go on some of his own. But one of his favorite stories was of the Impossible Girl, which he now knew was about his mother. She had been the one who told most of it. His father would sometimes make interjections that made Leo laugh.

He knew now that his mother had thrown herself into his father's time. His father had also told him a story of how she had once been in love with another man before him. That man was apparently one of the bravest men who had ever lived, and had loved Leo's mother so much that he saved the world and many lives for her at the cost of his own.

"Love is a promise," was what his parents had said to him when they told him they loved him.

He still believed that. He wondered if his parents did too. But no matter if they still did or not, that wasn't his focus at the moment. Saving Jemma was though. Leo wasn't sure he could live if she didn't.

* * *

Age: 27

Looking at the Monolith in the glass box made Leo's insides churn. Twenty-two years ago, he'd been pounding on it, screaming to be returned to his parents. For a while, he'd held onto hope that they would find him. Those days were long gone, and Leo knew he would never see them again.

But now, there was a chance he could get home on his own. He was far cleverer now, even with his accident, than he had been when he was five. If he figured out a way to open a portal with the stone, then he could get back to his own universe.

Jemma was currently beside it. She was his best friend, longest ally, and maybe something more. If he went through, then that would mean he wouldn't see her again. That was a flaw, unless…she wanted to come with him.

So he asked her to dinner. Mostly as a date, finally making a move and all that, but also to talk to her about the stone.

He intended to come back later, when no one was in the room, so he could get a better look at the stone.

The door to the chamber that held the Monolith was wide open when he returned, and Jemma was nowhere in sight.

When he rewound the security footage, Leo stared in horror as the rock turned to liquid and take Jemma. Everyone on the base came running when the scream left his mouth, all wondering what had happened. They were in just as much shock as he was after he showed them.

Why did he have to keep losing people that he loved?

* * *

Day 24:

The sensors started going off around the same area where they had lost Leo. Clara watched as the ground began to shift and the sands swirled into a portal of sorts. The Doctor was beside her, scanning the ground with his sonic to get a reading to figure out what exactly was happening. Both were hoping that their son would come through, safe and sound.

It was not Leo that came through, but a young woman. The portal spit her out onto the ground before closing. They watched her flail and scratch at the sand, as though trying to get back to where she came from. Eventually, she stood up and sighed something about trying to figure out where she was. Clara waited for her to see them, hoping that maybe this woman might have a clue as to where her little boy was.

"Hello!" she called out, drawing the newcomer's attention to her and the Doctor. "Are you okay?"

The woman looked up and down at them before shrieking and covering her mouth. "Oh my god, you're real! He was right!"

"Of course we're real," the Doctor scoffed as the woman approached them. "What did you think we were, cartoon characters?"

"I'm not dreaming, am I?" the woman muttered, pinching herself before looking back at the two of them. "Oh, I can't believe this."

"Who are you?" Clara asked. She was beginning to feel a bit unnerved by this woman's behavior.

"Dr. Jemma Simmons," the woman said, smiling brightly. "I'm with SHIELD. And you two…you're Fitz's parents."

Beside her, the Doctor stiffened. Clara inhaled sharply. Leo's middle name was Fitz. Did this Jemma know their son?

The Doctor was probably thinking the same thing. "As in Leopold Fitz Oswald-Smith?"

"He's never mentioned the Oswald-Smith part, but that last name makes sense," Jemma mused before her gaze darkened. "And how dare you. How dare you give up on finding your son!"

"We haven't!" Clara said defensively, crossing her arms across her chest. "And we never would. We have spent every day searching for him since we lost him. Twenty-three days, and you are the first person we've met who might have any knowledge of what happened to him!"

Jemma's lips parted as she looked between the two of them. "I'm sorry, but did you say he's only been missing for twenty-three days for you two?"

"It's been twenty three days on this planet and for us, yes," the Doctor grumbled as Jemma went pale. "You know our son, don't you?"

"I do," Jemma nodded, pulling a phone out of her pocket with shaking hands. The poor girl looked close to tears. "Time must pass slower in this dimension."

"Why do you say that?"

"Because maybe it's been twenty-three days for you, but it's been twenty-three years for your son," Jemma replied, holding out the phone to them. "This is Leo Fitz."

The Doctor took the phone and studied it. Clara watched as his shoulders sagged.

"Clara, it's him."

The phone was passed over to her. Clara looked down at the screen and instantly knew that the young man smiling in the picture was her Leo. She could see her son growing up into this man, but she'd missed out on it. Both she and the Doctor hadn't been able to watch him grow up. The past twenty-three days had been the worst in her life, feeling more like months than days. Now, knowing her son had been without them for twenty-three years, those days were much worse.

"He's all grown up," she whispered hoarsely, passing the phone back to Jemma and remembering something that she'd said earlier. "Does he really think we gave up?"

Jemma nodded. "He told me when we were twenty, just a few years after we first met. Fitz told me that he gave up hope you'd come back when he was about ten."

Clara covered her mouth with her hand.

"Well, he's not waiting any longer," the Doctor said from beside her. "The readings I got when you came through were better than I expected. I can figure out a way to reopen the portal, but we're going to have to start right now. Time is precious now."

"Every day here is a year back home," Jemma nodded. "At least that's what I've gathered so far."

"So let's get started so we can see our son," Clara smiled for the first time in days. "And in the meantime while we're working, you can tell us all about Leo, Jemma."

* * *

Age: 28

Leo sat in his room, staring at the picture of him and Jemma in Peru. He wished that he could go back to that moment, when everything was relatively peaceful and normal and nothing had gone terribly wrong. Back when they had been FitzSimmons and they still had the Bus and everything was as normal as it could possibly be before Hydra and everything else happened.

Someone started pounding on his door. "Fitz! Open up!"

Leo sprang off of his bed and bolted over to the door. On the other side was Mack.

"What's going on?" he asked, seeing that his friend's eyes were widened and he looked uneasy, which was very unlike him.

"The Monolith, it's…" Mack seemed to be thinking hard for a word. "Active."

"What?!"

"Just come with me. Something is going on."

Mack took off down the hall, and Leo sprinted after him. His hearts were pounding out of his chest as he thought of what this meant. Could Jemma be coming back? Maybe he could finally get back to his own world?

As he and Mack entered the room where the Monolith had been housed, Leo saw that Coulson, Bobbi, and Daisy were already there. All were staring at the stone, or at least what had been the stone. The rock seemed to have liquefied again and existed as a swirling vortex on the floor between the box and them. Leo noticed the doors of the containment unit of the floor, or at least the pieces of them.

"When did this happen?" he asked, staring at it.

"A couple minutes ago," Daisy answered, taking a step further back behind everyone else. She looked paler than normal. "Bobbi just walked by and told me she saw it like this, so I sent Mack to go get you."

"How did this happen? Did you all do something without me?"

"No," Coulson shook his head. "Nothing's been done at all since you last attacked it. It just happened out of nowhere."

Suddenly, someone shot out of the portal and landed on the ground to the right of it. The person groaned and climbed to her feet. Leo couldn't help but gasps when he saw it was Jemma. She looked exactly the same as she had six months ago, albeit slightly ruffled. It was like time had bared passed at all for her.

"Jemma?"

His best friend turned toward him, a smile breaking out over her face. "Fitz."

Leo ran past the team and embraced his friend. He didn't care if no one else was doing this. He had missed her too much to stop himself.

"I missed you," he murmured in relief as she returned the hug.

"I did too. Although you did have to wait longer."

Leo frowned and stepped back, confused. "What do you mean, wait longer?"

"It's been around six months here, right?"

Leo frowned and nodded, confused.

"I've only been gone for twelve hours," she replied before gazing back at the portal. "And they should be here by now…"

"Who should be here?"

Leo had forgotten that there were still other people in the room until Mack asked the question. He could see the others moving their hands toward their guns.

Jemma opened her mouth to explain, but then something happened. Two more people fell out of the portal a little ways away from where he and Jemma stood. Leo backed up a few steps, grabbing Jemma's hand and pulling her back as well. On the other side of the room, Mack, Bobbi, and Coulson all had their weapons out. Daisy was holding a gun too, but she was further back than the rest of them.

One of the newcomers rose to his feet and sighed in annoyance. "What is it with humans and guns?"

Leo froze as he heard the man's voice. He knew it all too well. It had been years since he had heard it in person, and he never thought he would ever again.

"Dad?" he uttered just as he saw the other person's face. "Mum?"

"Leo?" his mother was staring at him. "Is that you?"

He nodded and stepped toward them. They looked exactly the same as he remembered them. It was as though the twenty-three years had never happened to her. As he drew closer, Leo could see that she was starting to cry.

"The stone was a portal to your universe," Jemma explained beside him. "After I went through, I met them. They helped me get back here."

Leo was barely listening to her. He was too focused on his parents. His mother stepped forward, and Leo realized how much taller he was than his mother now. She tentatively reached out a hand to grasp his shoulder, as though she was trying to see if he was real. He could feel it shake before it steadied. Leo looked at her hand, and then back at her.

"Leo…" she murmured.

He swallowed, feeling every bitter thought he had had about being abandoned dissolve. "Hey, Mum."

She smiled and pulled him forward into a hug. The embrace was familiar, and Leo returned it after a few seconds, unable to believe it was real. Tears started to blur his vision.

A trembling hand settled on the back of his head. Leo looked up slightly to see his father joining the embrace. There were tears in his eyes too. Everyone in the hug was crying, and they all still were in various degrees when it ended.

His mother held him back by the shoulders, looking over him. She was still smiling, but it was a sad smile. "Look at you."

"Where were you?" he asked, unable to stop himself as the thoughts that had haunted him his whole life returned. "It's been twenty-three years."

"We know," his father nodded gravely. "Jemma told us about that."

"We're so sorry," his mother added.

"But you two look the same as I did when I was five. How is that possible?"

"Time in this universe moves much faster than the one we're from. A day there is a year here. For us, it's been twenty-three days."

Twenty-three days. They'd lost a five year old a little over three weeks ago and now he was an adult. "I thought you gave up."

"No," his mother shook her head. "We never gave up. We spent every day searching for you."

"It was days for you, but years for me!" he shouted. "I had hope that you were coming back, I did! But the longer I waited, the less hope I had that you were coming back! People mocked me when I told them you were coming to find me. They said you abandoned me, that you didn't love me. And the worst part was that I believed them!"

Leo sighed and took a deep breath, trying to calm himself down. "I figured out this was another universe a few years ago. Once I did, I assumed that you had realized that too and didn't want to get trapped here as well. That and you'd just given up on finding me."

There was a long, heavy beat of silence save for the noise of the still-active portal.

Finally, his mother opened her mouth.

"The moment you disappeared into that hole, it felt like time slowed down. Every day that went by felt like a lifetime for both of us. And every second, I felt that I had failed you. I have wanted to keep you safe since the day you were born, and the day I couldn't was my failure. I am so sorry, Leo. I should have done something that day, and I will regret not doing anything that day for the rest of my life. But you need to know that we never, ever gave up on you."

"I know, but were you going to?"

"Never," his father said. "I would have never stopped looking for you, and your mum wouldn't have either. We swore we do would do whatever it took to get you back. No matter how far we would have had to go, nothing would have stopped us. You are our son, and we'll always be willing to do whatever it takes if you are in danger."

"Because love is a promise," Leo murmured, remembering the words he'd been hearing since he was small.

"You remember that?"

He nodded and smiled at the both of them. "I should have remembered it more often."

His parents' smiles seemed happier now.

Across the room, someone coughed loudly. Leo whirled around to see everyone looking at Daisy.

"Hi," she said, giving a little wave. "Not interrupt this…family reunion, but would someone please explain to me what the hell is going on?"

"I'd like to know too," added Bobbi.

Leo looked between his parents and his friends. "Erm…"

"Fitz is actually from another universe," Jemma explained, stepping forward. "Time passes slower over there than it does here. I've only been gone twelve hours, although it's been six months here."

"Okay," Bobbi was nodding along with what the biochemist was telling her. "And the people with Fitz are?"

"My parents," Leo told her. "When I was five, I fell through a portal into this world. It's been twenty-three days since they last saw me, but twenty-three years for me. Every day there is a year here, right?"

His parents and Jemma nodded.

"Do they both have two hearts like Fitz?" Coulson asked.

Mack's eyes widened. "Two what?"

"You knew?" Leo stared at the Director.

"Fury told me personally when you were first assigned to the Bus," Coulson explained. "He thought I should know about that in the event that something happened to you that required medical attention."

"So there was another person who wasn't exactly human with us this whole time?" Daisy laughed. "Fitz, we could have started a club."

"I needed to keep it a secret. Imagine if Ward had found out about me."

The Inhuman shrugged. "Fair enough. But seriously, you have two hearts? Like the Doctor?"

Dad looked sharply at Daisy. "Did you know that? And how do you know I'm the Doctor?"

"There's a show?" she answered as his parents frowned. "You know, Doctor Who? Alien in a blue box travelling through time and space with a human companion. Saves the world or a planet or the universe?"

"That sounds like our lives," Dad muttered. "Why would anyone want to watch a show about my life or yours, Clara?"

"Doctor Who is actually reality over there," Jemma said to Daisy. "I'll explain more to you later."

"Fine," Daisy nodded. "So you came through a portal to find Fitz, and now you have. It's still open, so I'm assuming you're making a return trip. Are you leaving…with Fitz?"

All eyes turned to him.

"I think," his father said gravely. "That the choice is Leo's."

Leo felt his mouth go dry. The choice was all up to him. He could go home or stay in the world he had fallen into years ago.

"Excuse me," he muttered before turning and running out of the room.

Once he was outside, he ran down the hallway until he was around the corner. Leo stopped and sat back against the wall, letting out a ragged breath. He needed to make a choice that would impact the rest of his life. He was no longer a child who needed his parents. Twenty-three years spent here had made this world a sort of home. There were people who cared about him, like his adoptive mother and the team and Jemma. Going with his parents would mean leaving them behind in this world.

But this world wasn't truly his home. Home was a place that you missed when you left, and he had been missing the universe he had been born in for years. His family was there. His mother and father. The TARDIS. His aunts and uncles across time and space. Lucy was back there too, even though he was way older than her now.

He could go back, maybe travel with his parents for a while before settling down somewhere to build his own life. Maybe he could go work for UNIT…

"So what are you going to do?"

Jemma appeared around the corner just then. Her face was neutral, not giving any indication of what she wanted.

He shrugged his shoulders and stood up. "I don't know. I have two worlds to choose from, but I don't know what to do."

"Where do you want to go?"

"I want to go back to my own world, but-"

"So do it," Jemma interrupted. "Go home."

He looked at her. "What?"

"Go home," Jemma repeated. "Fitz, I know you want to. Ever since you told me about who you really were, I knew you wanted to be back in your own universe. I saw the look on your face when you talked about your parents. No matter how upset you were with them, you missed them, and you wanted them back. Now you know that they never gave you up, and that they want you back. This is your chance to go home. Don't miss it."

"She's right," Daisy said as she came around the corner. "Your parents are here. They came from another universe to get to you. And you have the chance to go home, to be a family again. If I had the chance to see my mother alive again, or talk with my father just once where he remembered who I was, I wouldn't turn it down no matter what they've done. So take some advice from me, and go home with them. You might never get another chance."

Both Daisy and Jemma were right, but there was still something that bugged him. "You understand I might never see you two again."

Jemma nodded slowly. "I know."

"Kinda figured as much when I asked earlier," Daisy mumbled beside her.

"Unless you want to come with me?"

Daisy chuckled sadly and shook her head. "Going near that portal made me feel physically sick. I'm not meant to go through. I guess it's sort of the universes' way of keeping Inhumans in this world."

Leo turned toward his best friend. She shook her head.

"I would love to, but it's not my world. It's yours though, and you've been away from it far too long."

She was right, unfortunately. Jemma normally was anyways. It would be unfair for her to be separated from her own world. But he did want to go home. He had missed his family. If he hadn't gone after that ball, he would have still been with his parents. But he couldn't go back, only forward.

"I think I'm going to go," he told them. "I just need to get a few things from my room. Most of it I can leave behind, but there's some things I won't leave."

"We'll give you some time," Daisy nodded. "C'mon, Jemma. Let's tell the others."

* * *

He ended up filling one backpack with the things he wanted to bring. His notebooks that he had written in calculations and designs for his inventions. Some knick-knacks he had picked up when they were still traveling around the world in the Bus. The ball that had led to him falling through the portal into this universe. There were photos too, all the ones he had with him. He didn't have a lot from when he was growing up, but he couldn't do anything about that now.

When he returned to the room where everyone was, his friends were standing by the door. They were all solemn, and he knew that Daisy and Jemma had told them his decision.

Coulson was closest, so Leo approached him first. "I'd like to announce my resignation from SHIELD, Director."

The older man nodded. "I understand, Agent Fitz."

"Fitz is actually my middle name," he blurted out, not sure why he was telling him this. "They just made it my surname after they found me on the street. I'm actually Leopold Oswald-Smith."

Coulson cracked a smile. "I accept your resignation."

"Thank you, sir."

"No, thank you, Leo."

He held out his hand. Leo shook it, smiling at the man who he had trusted for the past three years.

"Tell Agent May goodbye for me, will you?" he requested. "I wish I could myself, but just tell her thank you."

"I'll do that for you."

"Thanks."

Bobbi was the next goodbye. She stepped forward and hugged him before he could do anything.

"I'm going to miss you," she told him as he hugged her back before whispering. "We all will, but Hunter and Mack will probably miss you most of all."

"I'll miss all of you," Leo replied. "You're a good friend, Bobbi. Also, can you tell Hunter that he doesn't have to pay up on that bet anymore? And tell him goodbye too."

"I promise it'll be the first thing I'll tell him when he gets his ass back here."

"Thank you, Bobbi. Thank him too for me, for being a friend."

Mack was actually crying a little when Leo moved to him. He'd never seen his friend so upset before. It made Leo start to tear up again.

"Have a good life, Turbo," he said as he shook Fitz's hand.

Leo nodded and pulled his friend into a hug. He deserved one. Mack had been there for him after Jemma left to spy on Hydra. "You too, buddy."

"It's going to be a lot lonelier here without you around. I'm gonna miss having you around."

"I'll miss you too, Mack," he replied. "Thank you for being there for me when I needed it."

Daisy immediately threw her arms around him once he was done saying goodbye to Mack. Leo could feel something wet on his shoulder as she hid her face in it as they hugged. He'd already been crying before, but now the tears were starting to come faster. Daisy had been one of the first people to befriend him on the Bus. He understood her in so many ways, from her feelings about her parents and growing up without them to being different after San Juan. Leo wished he had told her about himself sooner.

Thinking about San Juan also made him wish he had been able to say goodbye to Trip too.

"Thank you for everything," she whispered to him. "You made me feel like a person again after everything that happened to me when I changed, and I needed that. You've always been there for me ever since I met you, and I wish I could repay you for everything you've done for me."

"You don't need to," Leo shook his head. "Knowing you, being your friend is enough. All that stuff, that's what friends do for one another."

Daisy pulled back and smiled at him. "So are you still going to be Leo Fitz back in your world?"

"Probably going to go back to Leo Oswald-Smith. Take back my old name. Little bit like you did."

"Telling you, we should have started a club," she laughed. "Goodbye, Leopold Oswald-Smith. Don't forget about me."

"Don't forget about me," Leo told her. "Goodbye, Daisy Johnson."

Jemma was his last goodbye. The person he'd said hello to first would also receive his final farewell of all his friends.

"So this is it then," she said as he approached her. "You're going home."

"Yeah. Sorry about the dinner."

"Me too. I'm sure it would have been lovely though."

"Yeah, it would have."

He stepped forward and hugged his best friend as tight as he could. He'd gone from thinking she hated him to befriending her to loving her. Now, he was saying goodbye to her.

"Thank you for finding my parents," he told her, trying not to cry too hard. "And bringing them here."

"I'm sure you would have done the same for me if our places had been switched," Jemma murmured back, her voice quavering a little. "Oh, Fitz, I'll miss you."

"I'll miss you too, Jemma. Can I ask you something-no, two somethings?"

She nodded against his shoulder.

"Can you tell Jane what happened?" he asked. "She took care of me all those years, and she deserves to know about this."

"I will, Fitz. And the other thing?"

"In Glasgow, there's a bakery, Florence's. A few blocks down, there's an abandoned apartment building. On the fifth floor, there's a door that looks like it's been boarded up, but it's not. I used to live there. Hopefully, my stuff is still there. I need to you to find the sonic screwdriver and lock it up somewhere here on the base. Don't tell anyone else about it, don't let SHIELD use it. Just keep it somewhere, okay?"

"Okay," she smiled at him tearfully.

"One more thing too," he added. "Have a wonderful life. Don't mope over me. Live your life, make it great. Do that for me, will you?"

"As long as you do, I will," she said.

"Goodbye, Jemma."

"Goodbye, Leo."

She kissed his cheek and stepped back. "Now go home."

Leo nodded and walked over to where his parents were standing by the edge of the portal.

"What happens once we go through?" he asked them.

"The machine that's keeping this open gets shut down," his father replied. "The portal between the universes will close, and that's it."

This really was it. The last time that he would ever see his friends, or at least the ones that were here right now. May and Hunter would never get a proper goodbye from him.

His mother turned toward the people he had known for the last three years. "Thank you for everything you've done for our son. And Jemma?"

The biochemist lifted her head.

"I know you didn't want to fall through that portal, but I'm glad that you did."

Leo watched his best friend smile through her tears and nod. "Me too."

"So," his father said. "Shall we?"

"How do we do this, just hop in?

"Worked well the first time. Ready, Leo?"

He gave a nod, looking back one last time at the team. Their faces were ingrained in his memory, and they always would be. He would never forget them.

"Good-bye," he said, taking a step forward into the portal, letting himself fall through.

* * *

One moment, he was in the Playground, and the next he was on the surface of the planet he hadn't seen in years. His parents came through just after him, and his father pointed his sonic at a strange box-like machine. The swirling portal behind them closed, leaving nothing but sand. It was closed.

Leo climbed to his feet and looked at the horizon. The sun was rising in the distance, bringing light to the surface of the planet. Looking to his right, he saw the TARDIS parked nearby the machine. Raising his hand, he snapped his fingers like he remembered seeing his parents do years ago. The doors swung open, and a smile spread across his face.

He was home.

* * *

 **So there you are. Probably one of the longest one-shots I have ever written. I am pondering a sequel too for it now, one that might involve the other child of this theory.**

 **Also, Aunt Nelly- totallt Petronella Osgood. I don't know why I did it, but I liked it.**

 **Thanks for reading, and I hope you'll leave a review behind!**


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